This invention relates to solenoids using ferromagnetic armatures subdivided into laminations to reduce eddy current losses. It relates more specifically to a lamination stacking geometry that combines good electrical/magnetic properties with high mechanical strength. It further relates to the use of stacks of slotted laminations, to provide an armature with high strength, reduced weight, high flux handling, and low eddy current losses. This invention is applicable especially to actuation solenoids for automotive engine valves.
Most solenoids are fabricated from iron or silicon steel alloys, where silicon alloying causes a large increase in electrical resistivity, which is traded off against a small decrease in flux handling capacity. Even with silicon steels, however, eddy current losses present significant performance problems in two broad classes of solenoids.
The first eddy-sensitive class is solenoids that are excited by AC rather than DC currents. AC excitation offers certain advantages, most notably, inductive self-limiting of current, so that an open AC solenoid pulls the high current needed to close, while the closed solenoid pulls a much lower current needed to maintain latching, the current reduction arising from the higher inductance of the closed solenoid. AC solenoids are generally constructed of laminations rather than solid metal, in order to reduce power dissipation by eddy currents and prevent overheating.
The second eddy-sensitive class is high performance solenoids that are excited by DC or pulse width modulated AC or DC and that are designed to move and be energized and de-energized very rapidly, often with a need for tight magnetic control or servo control of motion, and possibly actuated very frequently. Significant in this class are dual-acting solenoids used to open and close cylinder valves in automotive engines. Rapid energization and de-energization induces large eddy currents in unlaminated metal solenoids, with several adverse consequences. First is the matter of heating and power dissipation, which become significant for solenoids that are operated very frequently. Second is the dissipation-related issue of output capacity for the solenoid power supply and switching electronicsxe2x80x94capacity that must be increased to overcome eddy current losses. Third is the issue of response speed, which is slowed when eddy currents oppose the magnetomotive force of winding currents. Eddy current phase lag and reduced response bandwidth compromise both the speed and precision achievable with servo control.
While tubular solenoids and open-frame solenoids using a single bent piece of metal are common in DC and low performance applications, stacked laminations in an xe2x80x9cE-Ixe2x80x9d or xe2x80x9cU-Ixe2x80x9d configuration are typical of laminated designs, as illustrated respectively in FIGS. 1 and 2 by assemblies 101 and 201. The xe2x80x9cExe2x80x9d core yoke of FIG. 1 includes both E-shaped yoke laminations and a single electrical winding, 120, drawn with a smooth outer surface (e.g., a paper wrapping) and a circular or spiral pattern visible on the bottom of the winding. The xe2x80x9cUxe2x80x9d core yoke at 201 of FIG. 2 includes U-shaped laminations and two electrical windings, 220 and 225, shown surrounding the two legs of the xe2x80x9cUxe2x80x9d. These two windings are typically wired either in series or in parallel with reinforcing magnetomotive forces, promoting the flux loop through the xe2x80x9cUxe2x80x9d and xe2x80x9cIxe2x80x9d cores and across the gaps of width indicated at 240. The moving armature element in a laminated solenoid may consist of a stack of xe2x80x9cIxe2x80x9d laminations forming a flattened rectangle, e.g., armature 130 of FIG. 1 or armature 230 of FIG. 2. The typical mechanical solenoid configuration is similar to transformer configurations, except that in a transformer the xe2x80x9cIxe2x80x9d laminations are placed on alternating sides so that the xe2x80x9cExe2x80x9d or xe2x80x9cUxe2x80x9d laminations interleave with the xe2x80x9cIxe2x80x9d laminations. In a solenoid, the laminations do not interleave, and the xe2x80x9cIxe2x80x9d laminations are all stacked on one side as a moveable armature, as shown with 130 and 230, or else a solid slab of metal substitutes for the xe2x80x9cIxe2x80x9d lamination stack. Magnetic flux travels in a loop around the box formed by a xe2x80x9cU-Ixe2x80x9d pair of lamination stacks, as through yoke 210, across air gap 240, into armature 230, back across gap 240 on the opposite side, and returning to 210 to complete the circuit. As the armature moves axially to close gap 240, the reluctance of the magnetic circuit excited by windings 220 and 225 is reduced, reaching a minimum when the armature approaches or contacts the yoke, closing the magnetic circuit with minimal air gaps. In the case of an xe2x80x9cE-Ixe2x80x9d pair, the flux path describes a pair of loops, going through the center of the xe2x80x9cExe2x80x9d, e.g., of 110, across gap 140 to armature 130, splitting into separate paths to travel to the ends of 130, back across gap 140 to the outer fingers of 110, and completing the circuit as the separate flux paths converge back to the middle of 110. In either the xe2x80x9cU-Ixe2x80x9d or xe2x80x9cE-Ixe2x80x9d configuration, most flux completes a full loop within the plane of individual pairs of laminations of the yoke and armature. Eddy currents induced by such a flow of magnetic flux tend to circulate in a plane perpendicular to the direction of the B-field. Since the B-field itself flows in the parallel and typically flat planes of the laminations, the plane in which eddy current loops tend to circulate is chopped up by the laminations, as is desired so that the laminations inhibit the eddy currents.
The disadvantage of an armature consisting of a relatively deep stack of narrow xe2x80x9cIxe2x80x9d laminations is that it is inherently weak against bending moments in a direction tending to cause separation of the laminations. In the xe2x80x9cE-Ixe2x80x9d configuration of FIG. 1, it may be necessary to reinforce and strengthen the armature in various ways that add weight and, sometimes, introduce undesirable eddy current paths, partially defeating the function of the laminations. In engine valve solenoids, common practice has been to use a solid unlaminated armature, accepting the penalty in eddy current performance in order to achieve strength. Thus, there are inherent difficulties in achieving a mechanically robust armature using laminations to good advantage.
Note that the figures do not show components for coupling solenoid armatures to a mechanical load. Typically, a shaft would connect to, or penetrate through, the center of the armature lamination stack of FIG. 1 or of FIG. 2. The figures omit these details to focus attention on the configuration of magnetic lamination material.
The prior art offers examples of armature laminations stacked in a plane perpendicular to the axial direction of motion, but not in solenoids structurally or functionally similar to the present invention. As will be shown, the present invention relates to variable reluctance actuators in which an armature closes an axial magnetic gap with a yoke structure. Magnetic reluctance in such solenoids changes abruptly with the closure or near-closure of that axial gap, producing rapid armature flux changes acting strongly to produce eddy currents. It is characteristic of such solenoids to exert high forces over short ranges near closure, with highly nonlinear characteristics. It is also characteristic of such solenoids to produce high bending stresses in their relatively thin rectangular or disk-shaped armatures. In U.S. Pat. No. 4,395,649, Thome et al. illustrate a solenoid adapted for inducing vibrations, based not on axially disposed armature and yoke with a closing axial gap, but rather on radially-disposed armature and yoke with a non-closing radial gap. The variation of reluctance with armature position is smooth, not abrupt, avoiding the abrupt shifts in magnetic flux that tend strongly to excite eddy currents in Applicant""s context. Thome et al. do not discuss the relationship between lamination orientation and eddy currents. The armature taught by Thome et al. is a relatively deep cylinder, not a thin rectangle or disk, so that bending stresses in the armature are not an issue. In U.S. Pat. No. 6,013,959, Hoppie describes a linear motor whose principal mode of force generation is interaction of time-varying yoke magnetic fields with permanent magnet fields in the armature. Variable reluctance plays a minor role in Hoppie""s system, in contrast to Applicant""s system, which lacks permanent magnets and relies entirely on variable reluctance. Like the system of Thome et al., the moving armature laminations of Hoppie slide back and forth past the concentric edge of the stator, and these laminations are in deep cylindrical stacks axially supported by permanent magnets and end caps, so that bending stresses are not an issue. The choice to stack armature lamination disks axially appears to be at least partly a matter of fabrication ease, as noted by Hoppie in related U.S. Pat. No. 6,039,014, which states: xe2x80x9c . . . ideal laminations would be pie-shaped segments extending the entire length of the actuator. In practice, such laminations are difficult to produce.xe2x80x9d The same pragmatic concern probably motivates the structure of Thome et al.
It is an object of the invention to provide a solenoid armature made of laminations, such that the planes of the laminations lie flat in a plane perpendicular to an axial direction of motion of the armature. Laminations in such an orientation will henceforth be described as xe2x80x9cflatxe2x80x9d or xe2x80x9clying flatxe2x80x9d, phrases intended here to indicate an orientation perpendicular to an axis of armature motion, rather than simply describing the laminations as planar. A further related object is to make a flat lamination armature strong, to resist bending moments associated with axial forces of electromagnetic attraction and of mass acceleration and of pole face impact. A still further object is to orient laminations so that they inhibit induced eddy currents. To supplement the effect of flat laminations and inhibit eddy currents induced within a flat armature lamination plane by axial components of changing magnetic flux, it is an object to optionally provide slots in those laminations, especially in regions where there is a significant component of changing magnetic flux traveling through the thickness dimension of the laminations. A related object is to cause slots to fall into alternating positions for alternate laminations, so that an adhesive can bind all the laminations of an armature into a rigid solid containing isolated internal voids or separated slots that inhibit eddy currents and reduce weight while maintaining high mechanical strength. It is an object to shape and distribute slots so as to not reduce the flux handling capability of the armature. It is an object to employ flat laminations in armatures, possibly including slots, in conjunction with yoke geometries characterized by the descriptive phrases xe2x80x9cU-corexe2x80x9d and xe2x80x9cE-corexe2x80x9d and xe2x80x9cpot core.xe2x80x9d